In transporting perishable cargo, it is necessary to keep the cargo temperature controlled during the entire trip. Where, for example, a diesel engine is the power source for the refrigeration system, the diesel engine has the potential of incurring the same problems and difficulties of any diesel prime mover engine, such as overheating and lack of lubrication. Similarly, the refrigeration and heating system is subject to mechanical and electrical malfunction. Normally, in engine driven applications, indicator or "idiot" lights are used to monitor these conditions, but in the case of transport refrigeration equipment such as tractor-trailers, these lights are of limited value since the driver is remote and could be away from the equipment for extended periods of time. Since there is a high probability that no one will be in a position to heed a light's warning to shut down the equipment before major damage occurs, it is therefore common practice to eliminate these optical indicators and shut down the engine automatically in response to the sensing of an abnormal condition. The usual way to achieve shutdown to protect an engine in this type of application is to utilize electromechanical switches, commonly referred to as "safety switches", positioned on the engine, each sensing a particular problem. If a problem occurs, the appropriate switch opens and the engine is shut down.
The dilemma with this approach is that although the equipment is protected there often is no immediate way of discerning the reason for the shutdown. By the time an operator/driver discovers the shutdown, the water may have cooled, refrigerant pressure may have equalized or an electrical overload may have abated. Since a safety switch resets automatically, it is difficult or even impossible to pinpoint the instant problem.